Before I get to the possessive forms, I’d like to quickly review the rules on plurals. Today’s source material comes from Meredith College’s grammar page, found http://www.meredith.edu/grammar/plural.htm here.
The plural form is used when there is more than one of whatever it is that you’re talking about. To make most nouns plural, simply add an “s” to the end. If the singular word ends in s, z, ch, sh, or x, add “es” to the end. If the singular ends in y, drop the y and add “ies”. Irregular plural forms such as man/men, person/people, and species/species don’t conform to any rule and simply must be memorized. Never add an apostrophe when making a word plural.
The possessive form is used when whatever it is that you’re talking about owns or possesses something else. If the noun is singular, add an apostrophe and the letter “s” to the end. If the noun is singular and ends in an s, x, or z, add the apostrophe all by itself. Plural nouns that already end in “s” get only an apostrophe; plural nouns not ending in “s” get an apostrophe and “s”.
Sometimes you’ll have a sentence with a compound noun (more than one noun) as the subject of the sentence. This most commonly happens with people. If the people share joint ownership, then you only add the possessive to the second person, as in Jenna and Marie’s vacation reservations to Hawaii. If they each have individual ownership of similar items, add the possessive to both names, as in Marie’s and Jenna’s grades were similar over the course of their college careers. Marie’s grades are Marie’s, and Jenna’s are Jenna’s, and they are not necessarily the same grades, however, they share a single set of reservations for a trip to Hawaii.
Remember when to use an apostrophe this way: if something owns something else, then the sentence owns an apostrophe. If there’s no ownership involved, there should be no apostrophe involved.